This invention relates to magnetic bearings, and more particularly to position sensors of the type adapted to measure the position of a shaft rotatably supported in a housing with an accuracy of a few ten-thousandths of an inch.
As is well known, active magnetic bearings provide contact-free support of a rotating shaft by positioning an array of electromagnets about the shaft and controlling the current in the coils of the electromagnets to levitate the shaft at a position more or less centered within the array of electromagnets. Shaft position sensors, preferably also of the non-contact variety, are required to sense the position of the shaft and provide shaft position signals to a controller which regulates the current in the coils to achieve the desired shaft position. Typically, each radial bearing requires two position sensors. Active magnetic axial or thrust bearings, often used in conjunction with magnetic radial bearings, require an additional sensor. Thus, a machine with two sets of radial magnetic bearings and an axial magnetic bearing will require five shaft position sensors, all positionally associated in reasonable proximity to the electromagnets which they control.
Active magnetic bearings can be used in a variety of applications, and the air cycle machine is used as an example herein to demonstrate the rather severe constraints which are put on the components in general, and on the shaft position sensors in particular.
Air cycle machines are used on aircraft to cool and condition bleed air supplied by a gas turbine engine such as the main engine of the aircraft or an auxiliary power unit.
In an air cycle machine, one end of the rotatable shaft is connected to a turbine while the other end is connected to a load which is typically a compressor. The bleed air passes first to the compressor where it is further compressed, and heated as a result of the additional compression. After being cooled by a heat exchanger, the compressed air is expanded in the turbine and, as a result, is chilled to a very low temperature for use by the environmental control system of the aircraft to cool the aircraft cabin and avionics. The compressed air acting on the turbine rotates the shaft which, in turn, drives the compressor.
To support the shaft, an air cycle machine typically employs three bearings. Two of the bearings are radial bearings which support the shaft and the elements carried by the shaft. The third bearing is a thrust bearing which holds the shaft in a fixed axial position. For optimum performance, very small clearances must be maintained between the machine housing and the tips of the compressor and turbine blades. If the bearings permit more than just slight amounts of free play, the shaft will shift when loaded and will allow the blade tips to contact the encircling housing. With air cycle machines operating at speeds typically approaching and at times in excess of 100,000 RPM, successful operation of the air cycle machine requires relatively robust bearings capable of precisely maintaining the position of the shaft and having a relatively long service life.
Electronically controlled and electrically energized magnetic bearings are well-suited for use in an air cycle machine since they are non-contact type bearings and have a potentially long service life. In radial magnetic bearings, several electromagnets are angularly spaced around the shaft and, when energized, produce attractive magnetic forces which cause the shaft to levitate in free space and within the housing. A closed-loop electronic controller adjusts the current supplied to each electromagnet so as to change the strength of the magnetic forces in varying directions and in response to external forces acting on the shaft. Position sensors provide the necessary shaft position feedback information to the controller. Specifically, position sensors provide the controller with the radial position of the shaft in two orthogonal directions at locations near each of the radial bearings.
Eddy current position sensors are especially well suited for use with magnetic bearings. Small, precision eddy current sensors having a linear operating range of, for example, .+-.0,010' can be manufactured having a resolution of .+-.0.0005' for the precise control needed in a magnetic bearing application and will fit within a relatively small enclosure as might be encountered in an air cycle machine. Eddy current position sensors are non-contact type sensors and do not interfere with the operation of, for example, a high speed rotating shaft. Additionally, eddy current position sensors have no moving parts to wear out and potentially have a long service life, as do magnetic bearings.
During normal operation of an eddy current position sensor, a coil of electrically conductive wire located near the end of the sensor and adjacent the target, e.g., the shaft of the air cycle machine, provides the inductance portion of an LC circuit. The capacitance portion of the circuit is located in the sensor driver or controller. The controller drives the LC circuit at a relatively high resonant frequency to establish an oscillating electromagnetic field around the free end of the sensor, and monitors the voltage across the coil to determine the distance between the target and the sensor, the voltage being generally linearly related to the distance between the target and the sensor so long as the target is within the operating range of the sensor.
The accuracy, linearity, and overall performance of the sensor is related to the quality of the electromagnetic field. Generally, the quality of the field increases as the power losses in the coil decrease, for a given current supplied to the coil. Losses in the coil are due primarily to the resistance of the wire and to self-induced eddy currents in the wire, and to the skin effect. The resistance of a specific coil is related to the desired or required field strength of that coil. The AC losses, i.e., those due to self-induced eddy currents and the skin effect, however, can be reduced by decreasing the diameter of the individual wires which make up the coil. Litz wire, i.e., very fine wire carefully braided together, is well adapted to minimize AC losses, but is rather fragile.
Eddy current position sensors are not subject to mechanical wear, they are, however, subject to mechanical failure. Typically, the coil in a sensor is wound with relatively small diameter wire to minimize the self-induced eddy current losses. A wire of this size is relatively fragile and must be handled carefully. Mechanically stressing this small wire is carefully avoided during the winding of the coil and the manufacturing of the sensor. On the other hand, lead wires from the sensor to the controller must be relatively large and more robust so that the wires do not break during the course of normal handling and anticipated mishandling of the machine in which the sensor is mounted. To establish electrical continuity between the coil and the lead wires, the end portions of the coil and the lead wires are typically soldered together. As a result of the difference in size between the end portions of the coil and the lead wires, the substantially larger lead wires can induce mechanical and thermal stress into the fragile coil end portions, causing the end portions of the coil to break near the solder joint.
Prior sensors, as well as other small electromagnetic devices having a coil wound from relatively small diameter wire and having relatively large diameter lead wires connected to the coil utilize several techniques to minimize the stress on the small wire. For example, some prior sensors locate the solder joints in a small space below the coil and then tape or otherwise secure the joint into this space. Additionally, the solder joints in a prior sensor may be secured by filling the open space in the sensor with either a hard epoxy-based or a pliable silicone-based moldable compound. Despite these efforts to minimize the effect of joining the large wires with the small wires, breaking of the small wire due to stress near the solder joint remains one of, if not the primary source of mechanical failure in prior sensors.
The potential for failure of the small diameter wire is substantially increased when the sensor is used in a high temperature, high vibration environment such as might be encountered in an air cycle machine. A shaft rotating near 100,000 RPM can result in substantial vibration induced stress in the small diameter wire. The temperature of the bleed air in the compressor side of the air cycle machine can approach 500.degree. Fahrenheit. The temperature of the air exiting the turbine side of the air cycle machine can approach -10.degree. Fahrenheit. During start up of the air cycle machine, these operating temperatures are quickly achieved and the small diameter wire may experience substantial thermal shock or thermally induced stress near the solder joint. As a result, despite the position feedback accuracy that can be achieved with the use of eddy current sensors, prior sensors would have an extremely limited service life in an air cycle machine.